This proposal addresses the question of cellular differentiation of the airway epithelium. The tracheobronchial epithelium consists of several different cell types including secretory (goblet, serous, Clara), non-secretory (basal, intermediate, brush, ciliary), and endocrine (Kulchitsky). Little is known of the mechanisms that operate in the control of the differentiation of the various cell types or the effect that the environment may have on the mode of production and differentiation of these cells. It is not clear which is the stem cell (or cells) of the airway epithelium. Specifically, the work proposed will determine whether or not the various cell types are lineally descended from a common precursor or stem cell. Using two animal models (rabbit and ferret) to address these questions, specific monoclonal and polyclonal antibodies will be prepared to tag and identify the different tracheobronchial epithelial cells. Epithelial cells will also be fractionated by a density gradient centrifugation procedure and cloned in culture to determine which are stem cells or precursors to other types. These methods will allow unequivocal identification of the epithelial cell types, as they do not rely on the subjective limitations of morphological pattern recognition. These aims relate to the long-term objective of understanding the mechanisms of differentiation and cell turnover in the tracheobronchial epithelium in normal and diseased states in vivo.